It really depends on your budget. If you just want to try it out, I recommend the Wacom Bamboo. It’s small, but has all the features you need in a tablet and it’s not too terribly expensive, so if it’s not for you, you’re not out a crap load.
The next step is the Intuos series, which come in larger sizes, with more features. It also supports different advanced stylus, like the airbrush. I’m not certain a large pad is necessary for 3d modeling, but I do illustrations, so larger is generally better.
Lastly, there’s the Cintiq line, which has a built in LDC screen in the tablet. I was skeptical about this concept, since I was able to work just fine with a regular tablet. I got one and found that my work wasn’t any better, but I was able to do it much faster. If you get paid to do the work, the Cintiq is a must, and pays for itself in the time savings.
I got the Cintiq 12wx, because I wanted to be able to put the tablet in my lap; the larger ones were a bit impractical for that. It litterally paid for itself in saved time, allowing me to do more work, in the first week I had it.
There’s the basics. Hope that helps.